fix casing from Gimp to GIMP

Description

Fix casing from Gimp to GIMP

I have:

[x] 🤳 made sure that what I am adding is an app for end users, not a developer tool / library (no "wl-clipboard-rs")
[x] 🔗 checked that the link I am using refers to the root of the project (example, https://mpv.io) or GitHub repo if the first is not available
[x] 🤓 checked BOTH the name and the casing of the project(s) I am adding ("GNOME Terminal" and not "gnome-terminal", "bemenu" and not "Bemenu", etc.)
[x] 💣 checked that I am using spaces for indentation and that my levels are correct (no tabs!)
[x] ✋ checked that my section has the correct casing ("My section", and not "My Section")
[x] 📝 checked that the projects and / or the section are alphabetically sorted ("Clipboard manager" then "Color picker", "bemenu" then "Fuzzel")